Have you ever thought "I love the nostalgia value of old Human!Baby concepts such as maternal woman who dotes on Sam and Dean and sexy punk who Dean definitely fucks and has 14 criseses over but also that's kind of weird and Dean is the Impala"? If so, this is probably for you because that's approximately what I was thinking.
So, basically, I think the Human!Baby is played by Jensen, in that she has Dean's face, but nothing else really. Dean would immediately assume a hostile shifter, but nope! She's dressed in all leather of course, and in her jacket theres a hole in her pocket that left an army man stranded in it, and hell maybe the warding sigils as tattoos. However, despite the more "badass" attire she comes off as distinctly soft with TFW, and she is most maternal with Sam & Jack.
This has soooo much potential for an exploration tool of Dean's gender, particularly because she insists on Baby & she/her, but clearly is an extension of Dean, meaning Dean has to grapple woth some things. Namely, that in beating the shit out of Baby, that was probably just as bad as beating up himself in meaning, and that Baby was only home to Sammy in that, Dean was home to Sammy, and now Sam has to acknowledge that Dean did sorta raise him
Also, out of everyone, I think this would be the least weird for Cas. Even though he has been/is human he does still primarily recognise Dean by soul, whereas the humans are way more caught up on appearance. I think Jack would also be pretty cool about it, though confused.
I think Sam makes a snide little comment to Dean about regretting naming her Baby and now Dean has to choose between letting his snarky little brother be right or defending Baby.
Also, the potential of Baby having to ride in another car and being pissed about how intensely weird and wrong that is, very amusing.
I think Baby would make Sam sandwhiches with the crust cut off and dote on Jack like a toddler and she would drape others in her jacket for comfort (another manifestation of the protection she provided) but while looking like a 6'1 old school punk biker and I love that
Also thinking Dean is gonna feel out-masced at first, if not overall, but like buddy your car is GNC af.
I've played with the idea of Cas being jealous but actually I don't think he is, at all, but part of Dean is definitely jealous because Baby gets to look cool, be genderweird, and take care of everyone, and Dean is very jealous actually.
I do Not know how to clarify that Baby is a step off of Humanity similar to Jack and Cas but different without saying "Baby is ADHD coded" but if I wrote this, Baby would probably be ADHD coded
I came up with this concept on Baby's birthday, actually, and am only just now posting. If you do anything with this, please tag me, and do feel free to pester me to discuss it more